If you press the prone key and quickly hit the replace barrel key (default 6) your character will start to crawl while standing up. Your movement is restricted, you are unable to run, unprone, or jump. basically freezes your character other than very slow movements.
You can reproduce it simply by pressing alt followed quickly by 6 while you are still moving into the prone position.
You can reproduce it simply by pressing alt followed quickly by 6 while you are still moving into the prone position.